Many types of engine transmission have been developed in the past. One type is a continuously variable transmission which typically includes a toroidal drive having at least one pair of toric traction disks which often react upon each other and which are rotatably supported in a housing along an axis opposite one another to define a toric cavity between them. A motion transmitting traction roller is disposed in the toric cavity. The traction roller is frictionally engaged with the toric disks in circles of varying diameters depending on the transmission ratio, and is so supportive that it can be moved to initiate a change in the transmission ratio. A toroidal type continuously variable drive can have one or more cavities and may be used, for example, to form part of an infinitely variable transmission.
A common type of continuously variable transmission includes a toroidal drive having dual cavities which are defined by two torsionally coupled outboard traction disks which react upon each other and two inboard disks which are positioned between the outboard disks and also react upon each other. One dual cavity toroidal drive of the "off-center type" is disclosed in U.S. Pat. No. 5,368,529. An off-center toroidal drive is usually considered one having an included angle of less than 180.degree. between the traction contacts (i.e., where the roller contacts the disks). An on-center toroidal drive is usually considered one having an included angle of about 180.degree.. The included angle is the angle formed by the lines between the center of the toric cavity and the traction contacts on engaged disks. The usual method for transmitting power through a dual cavity design of the "off-center type" is to input the power to the two outboard disks and use parallel shafting and gearing to transmit power from the inboard disks. One gear mesh used to effect this parallel shafting is usually trapped between the inboard disks. Such a two-shaft system is bulky and difficult, if not impossible, to fit into the available space provided for the transmission of a number of vehicles. In addition, it is often necessary to return to the original center line when transmitting power. In the past, this has required a second gear mesh to be used, in addition to the gear mesh between the inboard disks. Single cavity toroidal drives are also known to take up more space than desired.
Therefore, there is a need for a toroidal type transmission capable of inputting and outputting power along the same axis without having to use parallel shafting. Such a co-axial drive transmission takes up less space than parallel shaft transmissions and can therefore be used in applications with tighter space constraints. In addition, it is easier and less expensive to package a co-axial drive transmission in a housing than it is to so package a parallel shaft transmission.